We propose to continue for a further two years the screening and followup study of IV drug users at risk for AIDS which was funded by NIDA in October 1986. They study serves as the principal HIV screening mechanism for drug users in treatment in San Francisco. At present, it also provides the only clinical referral available for seropositive drug users. The principal aims of the study are: 1) To examine the changing prevalence of HIV infection in intravenous drug users, in San Francisco as a whole and in different user groups, and in particular in different ethnic populations. 2) To study the association of HIV infection with demographic, behavioral and clinical characteristics of drug users, and to use HIV prevalence information to evaluate public health interventions aimed at stopping the spread of HIV. 3) To study HIV infection and its progression to clinical AIDS in a prospective study of 150 seropositive users and 100 seronegative controls.